The endoplasmic reticulurn (ER) is a densely folded stack of unit membranes, often with the appearance of being wrapped in concentric layers around the nucleus. The membranes of the ER have an "inside" and an "outside" and enormous surface area. One side of these folded ER membranes-the side facing the cytoplasm-is liberally peppered with ribosomes for protein synthesis. Other ribosomes are found floating loose in the cytoplasm.

Although the details are hard to see in any one micrograph, serial sections reveal that the highly folded ER actually is continuous with the outer cell membrane. In reality it is a folded membrane that encloses a labyrinth of deep cavities inside the body of the cell. The side of the ER that lacks ribosomes is topologically connected with the exterior of the cell, and the ribosome-containing side is everywhere in contact with the cytoplasm.

The ER also is connected to the nuclear membrane and the Golgi complexes. It provides channels for access from the cell surface to deep within its interior, and an exit route for small molecules produced in the cell. In addition to protein synthesis, the inner surface of the ER is the place where fatty acids are esterified to fats for storage in fat globules in the cytoplasm, where phospholipids and cholesterol are synthesized for use in membranes, and where sugars are polymerized to mucopolysaccharides for secretion between cells.